Accessing uiOutput Value On App Load - r

Question

To simplify this example I've only included the necessary code to describe the issue I'm having. Should it be required, I will include a fully reproducible example, but I have a hunch that this issue can be solved through theory alone by someone with more experience using Shiny. Basically, I've programmed a Shiny app in R which looks something like this:
ui.R
plotOutput(outputId = 'heatmap1', height = "800px")
uiOutput('selectStrains')
uiOutput('selectRegions')
server.R
output$selectStrains = renderUI({
checkboxGroupInput(inputId='strains',
choices=sort(colnames(mousedata)),
selected=colnames(mousedata))
})
output$selectRegions = renderUI({
checkboxGroupInput(inputId='regions',
choices=sort(rownames(mousedata)),
selected=rownames(mousedata))
})
# more code
output$heatmap1 = renderPlot({
input$recalculate
mousedatamat = as.matrix(mousedata[isolate(input$strains), isolate(input$regions)])
heatmap.2(mousedatamat)
})
problem:
My problem is that in server.R, when the app is first loaded, input$strains and input$regions are both NULL. Therefore, mousedatamat in server.R will be a 0x0 matrix, and the heatmap will be empty on the front page of the app, which makes for a pretty poor user experience. I don't know how the ui.R and server.R files interact when an app is launched, so I'm finding it difficult to debug. To make the plot show up I either need to click a recalculate button (input$recalculate), or resize the window (but only in the horizontal dimension for some reason).
To add more mystery to the mix, I have the same heatmap figure on page 2 (I'm using a navbarPage layout), which shows up on the app load when I navigate to that tab! It is the very same code, only instead it is assigned to output$heatmap2. When I navigate back to page 1, output$heatmap1 still does not display.
I've tried placing the calls to uiOutput above plotOutput in ui.R, but the main reason I don't know how to solve this I think is because I don't know much about execution flow when an app is started. Any ideas or information on this topic?